This invention pertains to the use of plural, superposed, different coatings on a substrate material to efficiently absorb solar radiation while suppressing thermal reradiation.
Efficient conversion of solar radiation into heat energy requires an absorber having a surface which absorbs much of the incident solar energy while losing very little heat by re-radiation. The absorber must also function for long periods exposed to the environment without degradation. An ideal absorber has properties such that it absorbs essentially all radiant energy having a wavelength less than about 2 .mu.m and emits no radiation having a longer wavelength. No one known material even approaches these requirements.
Composite absorbers consisting of multiple layers of different materials have been developed and are known as absorber stacks. These stacks typically consist of an outer absorbing layer and an inner reflective layer. In operation, solar radiation is absorbed by the outer layer and is converted into heat energy. The heat energy is conducted through the inner reflective layer into an appropriate working medium. Very little heat is lost from the absorber because it is essentially transparent to longwave thermal radiation and therefore cannot emit. Radiation of heat energy from the working medium is blocked by the reflective inner layer.
Absorber stacks having a variety of compositions have been reported. Hottel and Unger prepared absorber stacks by spraying cupric oxide on bright aluminum. This work was published as Hottel et al, The Properties of a Copper Oxide - Aluminum Black Surface Absorber of Solar Energy, Solar Energy, 3.3, 1959. Kokoropoulos et al, Selective Radiation Coatings: Preparation and High Temperature Stability, Solar Energy, 3:4, 1959, applied coatings of Cu0 and Co.sub.3 0.sub.4 on polished nickel, silver and platinum. Lowery, Selective Coatings for Collecting Solar Energy on Aluminum, NASA Tech. Brief 73-10527, Marshall Space Flight Center, 21 pp., April 1974, electrodeposited bright nickel and nickel black onto anodized aluminum. A summary of the application of thin film coatings in solar-thermal power systems was recently published as: Peterson et al, Thin Film Coatings in Solar-Thermal Power Systems, J. Vac. Sci. Technol., 12:1, 1975. Finally, Lowery in U.S. Pat. No. 3,920,413, discloses use of a layer of zinc interposed between an aluminum substrate and a reflective layer of bright nickel.